1. Field of the Invention
The present invention relates to a trouble determining apparatus for DC boosting circuit and more particularly to an apparatus for determining a trouble in a switching device of a chopper type DC boosting circuit.
2. Description of the Related Art
Conventionally, the chopper type DC boosting circuit has been widely used in which by turning ON/OFF a current flowing through a boosting coil from a DC power supply with the switching device, back electromotive force generated in a boosting coil is accumulated in a smoothing capacitor through a rectifying diode so as to boost the voltage of the DC power supply. In an electric power steering apparatus for assisting an operation of the power steering wheel using a rotation of an electric motor, for example, there has been proposed such a technology that by boosting the DC voltage of a vehicle loaded battery by means of the chopper type DC boosting circuit and then supplying that boosted voltage to the electric motor to drive the same motor, the amount of current supplied to that electric motor is reduced thereby achieving reductions in capacity of used wires (reducing the diameter of the wire) and size of the electric motor. The applicant of this application invented a chopper type DC boosting circuit suitable for such an electric power steering apparatus (see patent document 1).
[Prior Art]
JP 2003-89360 A (Page 9, FIG. 4) is incorporated herein by reference.
(Configuration of Conventional Chopper Type DC Boosting Circuit)
FIG. 4 is a circuit diagram showing a conventional chopper type DC boosting circuit 60 used in the electric power steering apparatus. The electronic control unit (ECU) 50 of the electric power steering system (EPS) includes a chopper type DC boosting circuit 60. The conventional chopper type DC boosting circuit 60 comprises a power relay RL, a smoothing capacitor C1, a noise filter capacitor C2, a boosting coil L, a transistor Q, a rectifying diode D and a boosting circuit control unit 62. The positive terminal of the vehicle loaded battery B is connected to the ECU 50 through a fuse F while the grounding terminal of the ECU 50 is connected to the negative terminal of the vehicle loaded battery B. The transistor Q which is a switching device is composed of N-channel power metal oxide semiconductor field effect transistor (MOS FET). As the power relay RL, it is permissible to use any one of an electromagnetic relay using an electric magnet and a non-contact relay using a semiconductor switch.
(Switching Device Trouble Determination in a Conventional Chopper Type DC Boosting Circuit)
If in the conventional chopper type DC boosting circuit 60, the transistor Q which is a switching device is in trouble, normal boosting operation is disabled. Thus, the trouble determination in the transistor Q is necessary. For the reason, the boosting circuit control unit 62 has a function for determining a trouble in the transistor Q. The troubles in the transistor Q include open trouble and short-circuit trouble. The open trouble refers to a trouble that a state between the source and drain turns open regardless of a gate voltage (even if a high level driving signal is applied to the gate in order to turn ON the transistor Q which is a N channel, the transistor Q keeps OFF). The short-circuit trouble is a trouble that a short-circuit occurs between the source and drain regardless of the gate voltage (even if a low-level driving signal is applied to the gate in order to turn the transistor Q of the N channel OFF, the transistor Q keeps ON).
(Determining Operation for Short-circuit Trouble)
The boosting circuit control unit 62 executes the determination operation on a short-circuit trouble of the transistor Q before starting the boosting operation by turning ON/OFF the transistor Q. That is, the boosting circuit control unit 62 first is turned ON the power relay RL and at the same time, is turned OFF the transistor Q and next detects a voltage VP1 at a connecting point P1. The boosting circuit control unit 62, if the voltage VP1 is less than a reference voltage VSa, determines that a short-circuit trouble occurs in the transistor Q and if the voltage VP1 is over the reference voltage VSa, determines that the transistor Q is not in short-circuit trouble. That is, if the transistor Q is in the short-circuit trouble, the voltage VP1 of the connecting point P1 is determined by the voltage VB of the vehicle loaded battery B, a resistance of the fuse F, a DC resistance of the boosting coil L and an ON resistance of the transistor Q, because the connecting point P1 is grounded through the boosting coil L and the transistor Q. Then, if the voltage VP1 in the case where the short-circuit trouble occurs in the transistor Q is experimentally obtained and the reference voltage VSa is set to that obtained voltage VP1, the short-circuit trouble of the transistor Q can be determined.
(Determining Operation of the Open Trouble)
After the boosting operation is started by turning ON/OFF the transistor Q, the boosting circuit control unit 62 executes the determining operation upon the open trouble of the transistor Q. That is, the boosting circuit control unit 62 first is turned ON the power relay RL and executes the boosting operation by repeating the ON/OFF operation of the transistor Q, and after that, detects an output voltage VO. If the output voltage VO is equal to the voltage VB of the vehicle loaded battery B and no boosting is carried out, the boosting circuit control unit 62 determines that the transistor Q is in the open trouble and if the output voltage VO is higher than the voltage VB, the boosting circuit control unit 62 determines that the transistor Q is not in the open trouble.
(Problems of the Conventional Determination on Switching Trouble)
The conventional determination on a trouble in the transistor Q by means of the boosting circuit control unit 62 has following problems.
1) When the transistor Q is in the short-circuit trouble, if the power relay RL is turned ON to determine a short-circuit trouble, a rush current flows through a path of vehicle loaded battery B—fuse F—power relay RL—boosting coil L—transistor Q. This rush current is a very large current value even if a noise filter capacitor C2 is provided. For the reason, there is such a problem that a circuit device (fuse F, power relay RL, noise filter capacitor C2, boosting coil L) through which the rush current flows is heated abnormally thereby leading to a trouble. For example, if a non-contact relay is used for the power relay RL, the semiconductor switch can be in the open trouble or short-circuit trouble due to the rush current. Because the power relay RL cannot be turned OFF if the semiconductor switch of the power relay RL is in the short-circuit trouble, the fuse F may melt down due to the rush current. That is, if the non-contact relay is used for the power relay RL, there may occur such double defects that the semiconductor switch of the power relay RL is in the short-circuit trouble and the fuse F melts down due to the rush current. Further, if an electromagnetic relay is used for the power relay RL, there is a fear that the relay contact may be fused due to the rush current. Because the power relay RL cannot be turned OFF if the relay contact of the power relay RL is fused, the fuse F may melt down due to the rush current. That is, if the electromagnetic relay is used for the power relay RL, double defects that the relay contact of the power relay RL is fused and the fuse F melts down due to the rush current may occur. If such double defects occur, which a troubled section exists in the power relay RL or the fuse F cannot be specified, thereby taking a long time for restoration from that trouble.
2) To determine an open trouble of the transistor Q, whether or not the output voltage VO rises over the voltage VB of the vehicle loaded battery is detected by executing the boosting operation by repeating the ON/OFF action of the transistor Q. At this time, to improve the determination accuracy on the open trouble of the transistor Q, it needs to be determined that the transistor Q is not in the open trouble first when the output voltage VO becomes higher than the voltage VB only by the amount corresponding to a margin of a detection error. Thus, the ON/OFF action of the transistor Q needs to be continued until the output voltage VO becomes higher than the voltage VB only by the amount corresponding to the margin of the detection error, so that it takes a long time to obtain a determination result on the open trouble of the transistor Q.